1. Field of the Invention
The present invention relates to a method of controlling the amount of energies to be imparted to a substrate and more particularly to a method of controlling the total amount of energies to be imparted to a substrate by emitting a plurality of pulse energies from a pulse oscillation type energy source wherein the energy is changed for each oscillation, e.g., a method of optimally controlling an exposure amount in an exposure apparatus which uses a pulse laser such as an excimer laser as an exposure light.
2. Related Background Art
Exposure apparatuses having a pulse laser light source such as an excimer laser as a light source for exposure have been developed. Generally, the amount of the pulse laser light is varied about .+-.10% for each pulse and the power thereof decreases in a short period and a long period. Therefore, in such exposure apparatuses as disclosed in, e.g., U.K.P. Nos. GB 2155647 B and GB 2196132 B, the amount of the pulse laser light is detected for each pulse and accumulated. And, pulse emissions are continued until the accumulated value reaches a desired value. Conventional exposure control methods are divided roughly into a correcting exposure type method as disclosed in e.g., U.K.P. Nos. GB 2155650 B and GB 2192467 B and an accumulating exposure type method as disclosed in, e.g., U.S. Pat. Nos. 4,970,546 and 5,097,291 wherein each of the energies of a plurality of pulse lights necessary for exposing a shot area on a substrate is made approximately to coincide with a predetermined average light amount. An example of the correcting exposure method will be described below.
As shown in FIG. 8A, when exposing the projected image of the pattern of a reticle by a plurality of pulse lights on a photosensitive substrate (semiconductor wafer coated with photoresist, glass plate, etc.), the exposure is performed in two stages in the correcting exposure method. In the first stage, coarse exposure is performed in which pulse lights RE with large energies are used to impart an exposure amount slightly smaller than a proper exposure amount to the photosensitive substrate. In the second stage, correcting exposure is performed in which pulse lights CE with small energies are used to impart a remaining exposure amount to the photosensitive substrate. That is, in the correcting exposure method, as shown in FIG. 8B, an integrated or accumulated exposure amount (exposure energy) smaller by .DELTA.E than the proper exposure amount EG is imparted to the photosensitive substrate by illumination of the plurality of pulse lights RE (coarse exposure). At this time, the remaining exposure energy .DELTA.E is set to be smaller than, e.g., the exposure energy of each of the pulse lights RE for the coarse exposure. Next, in the correcting exposure, an attenuator or a light reducing filter is used to attenuate the exposure energy of the laser light per a pulse and the attenuated laser light (pulse light CE) is illuminated on the photosensitive substrate for a few of pulses so as to correspond to the remaining exposure energy .DELTA.E. Thus, by the coarse and correcting exposures, the proper exposure amount EG is imparted to the photosensitive substrate. In this correcting exposure method, as the coarse exposure is carried out until the slightly lower level (EG-.DELTA.E) than the proper exposure amount EG, the total number of pulse lights for an exposure can be limited small, preventing the throughput from being lowered. Further, in the correcting exposure, the pulse lights CE with small energies are used for exposure, so that the finally accumulated exposure amount can be made to coincide with the proper exposure amount EG or can be set within a predetermined allowable error range.
Also, for the correcting exposure, e.g., a light reducing filter plate 1 as shown in FIG. 9A is used to attenuate the power of laser lights. The light reducing filter plate 1 is provided with six mesh filters 2A to 2F arranged on the peripheral portion thereof at 60.degree. intervals. The mesh filters 2A to 2F have different attenuating rates (transmittances) as shown in FIG. 9B. For example, the transmittance of the mesh filter 2A is 100% and that of the mesh filter 2B is about 50%. It is possible to set the power of a laser pulse light LB to a desired level by rotating the light reducing plate 1 around an axis la to dispose the desired mesh filter in the optical light path.
However, in the above correcting exposure method, in order to obtain a desired exposure control accuracy, one of the mesh filters 2A to 2F of the light reducing filter plate 1 is selected in accordance with the deficient exposure amount (.DELTA.E) at the time of the finish of the coarse exposure. Then, the light reducing filter plate 1 is driven to position the selected filter in the optical light path of the laser light. Therefore, it takes a long time for switching the transmittance of the light reducing filter plate 1, resulting in lowering of the throughput. Further, although the combination of the energy regulating amount by the correcting exposure and the number of correcting exposure pulses is determined at the time of the finish of the coarse exposure, the combination is determined based on a control map stored in a memory of the exposure apparatus. This control map is determined in accordance with the respective transmittances of the plurality of mesh filters of the light reducing filter plate 1. Therefore, there is inconvenience in that the control map needs to be formed for each exposure apparatus.